1. Field of the Invention
The present invention relates generally to gyroscopic reference systems of the type including a plurality of strapped-down force rebalanced gyroscopic rate sensors for measuring craft rotation rates about its primary axes, together with a digital computer for computing from such measures aircraft stabilization and attitude data, for example. More specifically, the invention relates to rate range switching and control apparatus for adapting the full range of the gyroscope analog rate output signal to a range consistent with the range of precision conversion of the analog signal into a corresponding digitally compatible signal, for example, consistent with the precision range of a voltage-to-frequency converter. The invention comprises circuit apparatus for providing a rate tracking gate voltage for protecting the solid state switching devices of the system from destructive voltages.
2. Description of the Prior Art
Strapped-down gyroscopic reference apparatus for aircraft and space vehicles is well known to those skilled in the art of gyroscopic aircraft control systems and many gyro configurations and control systems based thereon have been described extensively in the literature. In general, such systems include a plurality of single-degree-of-freedom or two-degree-of-freedom rate sensors strapped down to the vehicle (usually skewed with respect to the vehicle primary coordinate axes for redundancy purposes) for measuring the angular velocities of the vehicle about its primary coordinate axes which data, along with vehicle acceleration and heading data, is supplied to a digital computer to provide output data for use in stabilization, control, navigation, and guidance of the aircraft. Since the gyros are strapped to the airframe, it will be appreciated that the rate sensors will be of the force or torque rebalance type; that is, the gyro is maintained substantially aligned with its support case by feeding the gyro pick off signal back to the gyro torquer in a manner to maintain the pick off signal essentially null, the torquer current so required being a measure of the rate being sensed by the gyro. A typical two-degree-of-freedom rate sensor of this type is disclosed in the present assignee's U.S. Pat. No. 3,529,477.
It will be appreciated that the ultimate output signal of the sensor must be compatible with the requirements of digital computation. One way of accomplishing this might be to convert the gyro pick off signal, for example, to a duty-cycle-modulated square wave of a suitable frequency and to apply this signal to the torquer and together with suitable clock pulses provide a frequency count proportional to sensor rates. Another way would be to convert the gyro pick off signal to a proportional direct current and then to apply this current through the torquer to a precision resistor being used to produce a corresponding voltage and to apply this voltage as an input to a voltage-to-frequency converter to provide a frequency proportional to the sensed rate. The present invention relates to the latter conversion technique.
During normal operation of commercial aircraft, body rates are relatively low, being of the order of 0.degree. to 30.degree. per second. However, flight safety considerations dictate that the rate gyro be capable of measuring body rates greater than the aircraft design maximum; such rates may be on the order of 150.degree. per second or more for commercial aircraft. Of course, military aircraft are designed to be capable of body rates of several hundred degrees per second and the strapped down rate gyros must be capable of sensing such rates for these applications. Thus, the range of torque feed back currents and the corresponding sensed voltages may be quite large. Also, the conversion precision of a voltage-to-frequency converter is, within reasonable circuit complexity and cost restraints, limited to a relatively narrow input voltage range, substantially smaller than the range of voltage corresponding to the full gyro torquer current range, so that if the full range of voltage resulting from the torquer current were applied to this converter it would saturate at the higher voltage levels.
The present invention overcomes the above conversion range deficiency by a unique switching circuit responsive to the gyro pick off signal for switching the torquer current between different voltage sensors so as to maintain the input voltage to the converter within the converter's precision range. Actuation of the switching means also alerts the digital computer that the range scale factor has been changed.
Depending upon the design of the rate range switching and control system, the solid state switching means may have to switch relatively high currents very rapidly, which factor severely limits the choice of available switches. For example, a switch that can handle the high currents may be restricted in terms of switching voltage; i.e., if the effective turn-on voltage exceeds a predetermined value, the switch will be destroyed. The present invention overcomes this difficulty by rate tracking circuit means which maintains the effective switch voltage substantially constant over the range of gyro rates.